1. Field of the Invention
The present invention relates to a battery charging apparatus for controlling the charging of a battery and/or the temperature of the battery, and more particularly to a battery charging apparatus suitable for use on an electric vehicle that is propelled by electric energy supplied from a battery as an energy source.
2. Description of the Related Art
As well known in the art, while a battery is being charged, the temperature of the battery increases due to a chemical reaction that occurs in the battery solution. When the battery temperature rises above a predetermined value of +50.degree. C. (which varies depending on the type of the battery), the battery suffers an irreversible deterioration in the form of a reduction in the capacity of the battery.
In order to avoid such a temperature-dependent irreversible deterioration, there have heretofore been proposed two temperature control processes that can be carried out during the charging of a battery.
According to the first temperature control process, when the temperature of a battery rises above a predetermined value while the battery is being charged, the charging of the battery is temporarily stopped, and the battery is cooled by ambient air. When the battery temperature drops below the predetermined value, the charging of the battery is resumed.
According to the second temperature control process, when the temperature of a battery rises above a predetermined value while the battery is being charged, a forced-cooling unit is operated to cool the battery with forced air, and the battery is continuously charged.
The first temperature control process is effective to prevent the battery from being deteriorated due to a rise in the battery temperature. However, from the standpoint of a battery charging efficiency (a portion, stored as a charge in the battery, of a charging current that is supplied to the battery), the first temperature control process suffers the following problems:
As the battery temperature goes higher and the battery is charged more closely to the fully charged state, the battery charging efficiency becomes lower, making the battery chargeable less efficiently. When the battery charging efficiency is lower, excessive electric energy is consumed by an electric decomposition of the electrolytic solution of the battery, resulting in an increase in the battery temperature. Stated otherwise, when the battery is charged more closely to the fully charged state, i.e., when the SOC (State of Charge) of the battery is high, the battery is charged at a high temperature, and a large amount of charging electric energy is wasted.
The second temperature control process allows the battery to be continuously charged, but is not optimum from the standpoint of a battery charging efficiency because the temperature at which the forced-cooling unit is turned on is the same as a constant temperature set for preventing the battery from being deteriorated.
Nickel-hydrogen secondary cells of a battery exhibit so-called soak-back characteristics according to which the battery temperature rises after it the battery has stopped being charged. The temperature rise after the battery has been charged cannot be prevented by the first and second temperature control processes.